1. Field of the Invention
This invention relates to a nitrogen oxide removal catalyst which is useful for the removal of nitrogen oxides present in combustion exhaust gases such as exhaust gas from boilers.
2. Description of the Related Art
Conventionally, in order to remove nitrogen oxides (hereinafter referred to as NO.sub.x) produced in boilers and various combustion furnaces for the purpose of preventing air pollution, a catalytic ammonia reduction process wherein ammonia is used as a reducing agent and nitrogen oxides are catalytically decomposed to nitrogen and water with the aid of a catalyst is being widely employed. Most of the NO.sub.x removal catalysts currently used for practical purposes are honeycomb-shaped catalysts which have through-holes of square cross section in order to prevent clogging with dust present in exhaust gas and increase the gas contact area. With respect to catalyst components, titanium oxide is highly suitable for use as a principal component, and vanadium, tungsten and the like are used as additional active components. Thus, TiO.sub.2 -WO.sub.3 binary catalysts and TiO.sub.2 -V.sub.2 O.sub.5 -WO.sub.3 ternary catalysts are popularly used.
However, when a NO.sub.x removal catalyst is used for the treatment of combustion exhaust gas, its NO.sub.x removal power tends to be gradually reduced. Of various NO.sub.x removal catalysts, those used for the treatment of exhaust gas from oil-fired boilers pose no problem because their reduction in NO.sub.x removal power is insignificant. In contrast, NO.sub.x removal catalysts used for the treatment of exhaust gas from coal-fired boilers show a considerable reduction in NO.sub.x removal power with the lapse of time, and hence require some counter-measure. In order to maintain the required NO.sub.x removal power, it is necessary to replace the spent catalyst with a fresh one. However, this is disadvantageous from an economic point of view because a fresh catalyst is expensive and the spent catalyst requires a cost of disposal.
Accordingly, an object of the present invention is to effectively utilize a spent solid type (i.e., the type in which the catalysts themselves form a shaped body) honeycomb-shaped NO.sub.x removal catalyst for use in a catalytic ammonia reduction process (in particular, a spent NO.sub.x removal catalyst which has been used for the removal of NO.sub.x from the exhaust gas of a coal-fired boiler) and thereby provide an inexpensive catalyst having NO.sub.x removal power equal to that of a fresh catalyst.